1. Field of the Invention
The present invention relates to an element driving device for matrix drive of multiple active elements, and more particularly to an element driving device for driving multiple active elements with a variable drive current.
2. Description of the Related Art
Heretofore, actively operated and controlled elements have been used in various devices. For example, an image display device utilizes a display element such as a light emitting element as an active element. Such light emitting elements include an EL (Electro-Luminescence) element and the like. An EL device includes an inorganic element and an organic element.
Inorganic EL elements have been put into practical use, for example, as back-lights of a liquid crystal displays, because they can realize uniform surface emission with reduced power. On the other hand, organic EL elements have not yet been developed sufficiently and have problems to be solved such as durability. However, there has been a demand for a practical use of organic EL elements because it can be driven with a low voltage direct-current, provides high brightness with a high efficiency, and exhibits a favorable responsibility.
Since the organic EL elements are driven with a current as described above, an element driving device differs in structure from that for a conventional inorganic EL element which is driven with a voltage. For example, Japanese Patent Laid-open Publication No. 54835/1996 discloses an element driving device for driving a light emitting element of current control type on an active matrix scheme.
This element driving device, however, is designed to control gradation of organic EL elements through turning on and off a plurality of transistors. Accordingly, numerous transistors are required for providing representation with multi-level gradation, which is not suitable for practical use.
Also, Japanese Patent Laid-open Publication No. 74569/1993 discloses an element driving device for driving an inorganic EL elements with a voltage. In the element driving device disclosed in the publication, a power supply electrode to which a predetermined drive voltage is to be applied is connected to an inorganic EL element through a TFT (Thin Film Transistor).
The TFT converts a drive voltage to be applied to the power supply electrode to a drive current corresponding to a control voltage to be applied to a gate electrode and supplies the drive current to the inorganic EL element. To control the amount of the current to be supplied, an element for holding a voltage is connected to the gate electrode of the TFT.
The control of emission brightness of the inorganic EL element is effected by controlling the voltage to be held in the element. Therefore, there is no need to increase the number of transistors for the increased number of levels of gradation in element unit as in the device disclosed in the aforementioned Japanese Patent Laid-open Publication No. 54835/1996.
An element driving device, the element driving device the aforementioned structure to the organic EL element which is an active element of current control type, will be hereinafter described with reference to FIG. 1 as one prior art.
An element driving device 1 illustrated by way of example has an organic EL element 2 as an active element and a power supply line 3 and a ground line 4 as a pair of power supply electrodes. A predetermined drive voltage is applied to power supply line 3, and ground line 4 is grounded.
Organic EL element 2 is connected directly to power while supply line 3 is connected to ground line 4 through TFT 5. TFT 5 converts a drive voltage from power supply line 3 to be applied to ground line 4 into a drive current corresponding to a control voltage to be applied to a gate electrode to supply the current to organic EL element 2.
The gate electrode of TFT 5 is connected to holding capacitor 6 for holding a voltage, which is also connected to ground line 4. Holding capacitor 6 and the gate electrode of TFT 5 are connected to signal line 8 serving as a signal electrode through switching element 7. A control terminal of switching element 7 is connected to control line 9 serving as a control electrode.
Holding capacitor 6 holds the control voltage to apply the voltage to the gate electrode of TFT 5. Switching element 7 turns on and off the connection between holding capacitor 6 and signal line 8. Signal line 8 is supplied with the control voltage for driving the emission brightness of organic EL element 2. Control line 9 is applied with a control signal for controlling the operation of switching element 7.
Element driving device 1 with the aforementioned the structure is capable of controlling organic EL element 2 at variable emission brightness, in such a way as to apply a control signal to control line 9 to turn on switching element 7, and with this state maintained, to supply a control voltage corresponding to the emission luminescence of organic EL element 2 from signal line 8 to holding capacitor 6 to hold the voltage.
The control voltage held in holding capacitor 6 is applied to the gate electrode of TFT 5, causing TFT 5 to convert the drive voltage constantly applied to power supply line 3 to the drive current corresponding to the voltage at the gate to be supplied to organic EL element 2. This operation continues even after switching element 7 has been turned off with the control signal applied to control line 9.
The drive current which is converted by TFT 5 from the drive voltage to be applied to power supply line 3 and supplied to organic EL element 2 corresponds to the voltage to be applied from holding capacitor 6 to the gate electrode of TFT 5. Therefore, organic EL element 2 may emit light at brightness corresponding to the control voltage supplied on signal line 8.
The aforementioned element driving device 1 is intended to be utilized actually as an image display device. In such a case, (m.times.n) organic EL elements 2 are arranged in m rows and n columns, m signal lines 8 and n control lines 9 are applied with the control voltage and the control signal respectively in matrix form, to make (m.times.n) holding capacitors 6 to individually hold the control voltages.
This causes (m.times.n) TFTs 5 to apply to each of (m.times.n) organic EL elements 2 as the drive current corresponding to the voltage held in each of (m.times.n) holding capacitors 6. As a result, organic EL elements 2 are allowed to emit light at individually different brightness, thus displaying an image in dot matrix form with gradation in pixel unit.
In element driving device 1, TFT 5 can generate the drive current to be supplied variably to organic EL elements 2 from the drive voltage to be supplied to power supply line 3. The drive current to be generated by TFT 5 from the drive voltage can be controlled with the voltage held in holding capacitor 6 which in turn can be controlled with the control voltage supplied to control line 8.
However, when the aforementioned image display device is manufactured actually using element driving device 1, each of m signal lines 8 may be connected to n of (m.times.n) organic EL elements 2. In connecting signal lines 8 of microstructure to a number of organic EL elements 2 to configure a high-definition image display device, the drive voltage to be supplied to organic EL element 2 would be varied due to a voltage drop on signal line 8.
Also, even if holding capacitor 6 is made to hold a desired control voltage to supply the drive voltage, the drive current supplied to organic EL element 2 will not correspond to the control voltage because the operational characteristics of multiple TFTs 5 of microstructure are not constant due to a manufacturing error. In this case, organic EL elements 2 in element driving device 1 are unable to emit light at the desired brightness, which results in deterioration of the quality of a displayed image with gradation obtained by an image display device using element driving device 1.